1. Field of Invention
The present invention relates to an optical position measuring instrument for detecting the relative position of a scanning unit, as well as a scale, which can be moved in at least one measuring direction in relation to it, wherein the scale includes at least one measuring graduation extending in the measuring direction, and at least one scanning graduation is arranged on the side of the scanning unit.
2. Discussion of Related Art
In known incremental optically based position measuring instruments, a scale with a periodic measuring graduation is typically scanned by a likewise periodic scanning graduation. The latter is arranged together with further components, such as a light source and a detector array, in a scanning unit that is movable relative to the scale in at least one measuring direction. In the ideal case, there is an exact sinusoidal scanning signal or incremental signal, which can be further processed—or further subdivided—in suitable follow-up electronic devices using known interpolation processes. Typically, the form of the detected scanning signals that actually results, however, deviates more or less sharply from the ideal sinusoidal form. The most various causes may be responsible for this, such as deviations in the graduation periods on the scale and/or on the scanning side from ideal conditions, imprecise edges of the graduation markings, and so forth. The scanning signals that actually result are therefore in practice burdened by harmonics, which in particular can impair the further processing of the signals in the form of the interpolation. It is therefore a fundamental goal to minimize the harmonic content of the scanning signals as much as possible.
Such harmonics result in connection with the most various optical scanning principles, or in other words both in conjunction with shadow-casting scanning principles and in the case of high-resolution, interferential scanning principles, or so-called grating-interferometric position measuring instruments. Harmonic filtering approaches for the last-named grating-interferometric position measuring instruments have already been disclosed in EP 547 270 B1. This reference proposes various variants for harmonic filtration, in each of which a specially embodied graduation structure is provided on the side of the scanned measuring graduation. Thus, the graduation areas located periodically in the measuring direction have a defined periodic transversal structure. To that end, the graduation areas have modulated contour shapes along their longitudinal extension direction. The exemplary embodiment, shown in FIG. 5 of the aforementioned reference, of this kind of measuring graduation is also known as a “fish-eye grating”. Although a measuring graduation embodied in this way assures adequate harmonic filtration, it nevertheless makes stringent demands with regard to the proposed contour shapes of the production of the applicable measuring graduation structures. For instance, in the embodiment of the contour shapes that taper to a point, very fine structures must be exactly transferred, which makes for complicated and expensive production of the measuring graduation. This is all the more serious if in this kind of harmonic filtration the filtering structure is not limited to a small scanning plate but instead is provided on the side of the measuring graduation. The measuring graduation may then have a length of several meters, so that for this reason if no other, stringent demands must be made of the homogeneity of the production process for the measuring graduation.